1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, or a facsimile receiver, and more particularly, to an improved image forming apparatus which has a developing unit for rendering a latent image visible through use of a two-component developing agent consisting of a carrier and toner and has toner supply means for supplying toner to the developing unit.
2. Description of the Related Art
An electrophotographic image forming apparatus is taken as an example of a common image forming apparatus, such as a copier, a printer, or a facsimile receiver. In a known type of electrophotographic image forming apparatus, a latent image is formed on an image carrier, such as a photosensitive drum, and a developing unit renders the latent image visible.
Various types of development methods have hitherto been employed by developing units. A typical development method having a superior development property employs a development roller disposed opposite the image carrier. A two-component developing agent mainly consisting of toner and a carrier is applied to the development roller by means of a magnetic brush. The developing agent having the form of a magnetic brush is located in close proximity, to (in a non-contact manner) or so as to come into contact with he image carrier, whereby the developing agent renders visible the latent image formed on the image carrier.
In the two-component developing unit using a two-component developing agent of this type, toner is consumed every time a developing operation is performed, thereby inducing a decrease in the amount of toner supplied to a development region at which the image carrier opposes the development roller. Thus, replenishment of toner is required in association with consumption of toner.
There has already been proposed a toner replenishing control method (described in, for example, Japanese Patent Publication 16199/1968). According to this method, a reference patch potential of an image carrier, for example, is detected from the density of a developed image through use of an image density detector. If the result of detection shows that the density of the image is less than a predetermined density, replenishment of toner is performed.
A developing agent supplied to the development region for developing purpose requires that a development roller be frictionally electrified sufficiently and uniformly in the axial direction thereof. A two-component developing unit is usually provided with an agitation-blending mechanism for sufficiently agitating and blending toner and a carrier stored in the developing unit.
In a case where a plurality of items of printed matter containing high-coverage images are continuously produced, the two-component developing unit of this type must be newly replenished with a large quantity of toner. Electrification of newly-supplied toner lags behind a development operation. As a result, development of a latent image and transfer of a developed image are performed through use of insufficiently-electrified toner, potentially leading to a fear of various detriments, such as, inconsistencies indensity, occurrence of a toner dust cloud, fog, or deterioration of image resolution.
Particularly, in a case where images to be continuously printed are smaller than the full length of the development roller in its axial direction (i.e., images are narrower than the maximum print size), consumption of a developing agent arises locally with reference to the axial direction of the development roller. As a result, a steep gradient of toner density arises across the length of the development roller with reference to the axial direction thereof, thereby deteriorating uniformity of image density.
As the technique for resolving the drawback. of the background art, there has already been proposed a method for realizing he uniform distribution of toner and uniform frictional electrification, by means of interrupting an electrophotographlc image formation process on the basis of, for example, an image area coverage; performing operation pertaining to a cycle of activating and mixing an extra developer; and blending and agitating a two-component developing agent.
According to the method of this type, in a case where high-coverage images are continuously printed, an electrophotographic image formation process is temporarily interrupted, and replenishment of toner and agitation and blending of a developing agent are sufficiently performed. As a result, density of toner and the electrification characteristic of toner are maintained properly, thus enabling sustainment of good picture quality.
This prior art encounters difficulty in solving the following technical drawbacks.
The above-described technology is based on the premise that toner is supplied in accordance with the size of the image coverage area. There is a potential fear that the quantity of toner might change in accordance with a change in the development property of a developing agent associated with a change in the electrification characteristic of toner due to a change in environment (changes in temperature and humidity) and deterioration of a developing agent.
A commonly-employed method is for controlling a toner supply volume by means of controlling the period of time during which a toner supply unit is activated. Further, there is a potential fear that the quantity of toner supplied per unit time may change in accordance with a change in environment or the amount of toner remaining in a toner storage tank, such as a toner cartridge.
A conceivable method of ensuring sufficient image density and good uniformity of image density across the length of the development roller is to set a smaller image area coverage beforehand in expectation of a variation in the quantity of toner consume or the quantity of toner supply or to set a shorter time period after which the process of forming an electrophotographic image is interrupted (the number of images to be continuously produced).
In this case, productivity of an image forming apparatus (the number of images formed per predetermined time period) is considerably deteriorated.
The present invention has been conceived to solve the foregoing technical problems with the background art and is aimed at providing an image forming apparatus which can maintain the excellent development property of a two-component development unit at all times without involvement of an unnecessary decrease in productivity even in a case where images of high coverage are continuously formed.
To achieve the above object, according to the present invention, there is provided an image forming apparatus including latent image formation means for forming an electrostatic latent image on an image carrier and a developing unit for rendering the electrostatic latent image visible through use of a two-component developing agent consisting of at least a carrier and toner, the apparatus comprising:
toner supply means for supplying toner to the developing unit;
toner supply control means for determining the final amount of toner supply, on the basis of the standard amount of toner supply corresponding to image coverage measured by image coverage measurement means and a change in the amount of toner supply corresponding to the concentration of an image detected by image concentration detection means 6, thereby controlling toner supply means;
toner supply history storage means for storing a history concerning the amount of toner to be supplied by the toner supply means;
toner consumption status determination means for determining whether or not an excessive amount of toner is consumed, on the basis of the toner supply history information supplied from the toner supply history storage means; and
development property stabilization means which interrupts formation of an image when the toner consumption status determination means determines that an excessive amount of toner is consumed, thereby causing the toner supply means to supply toner and causing the developing unit to agitate and blend a developing agent or to temporarily agitate and blend a developing agent.
The latent image formation means is not limited to latent image formation means of electrophotographic type, so long as latent image formation means can form an electrostatic latent image on the image carrier. Latent image formation means of another type, such as latent image formation means of electrostatic recording type, may be selected as the latent image formation means, as required. Further, as required, any developing unit may be selected as the developing unit, regardless of whether the developing unit is of monochrome type or color type or whether or not the developing unit is of trickle type.
When replenished with toner, the developing unit of two-component development type must electrify new toner with a predetermined voltage. The developing unit is usually provided with a developing agent agitation-blending mechanism for agitating and blending an existing developing agent and newly-supplied toner.
The toner supply means is not limited to a cartridge type; any type of toner supply means may be selected, as required, so long as the toner supply means replenishes the developing unit with toner.
The toner supply control means may adopt any algorithm, so long as determination of the final amount of toner supply can be effected through use of the standard amount of toner supply corresponding to image coverage and a change in the amount of toner supply corresponding to the concentration of an image.
The image coverage measurement means for measuring image coverage may be arranged so as to measure image coverage on the basis of an electrostatic latent image formed on the image carrier or a photo image formed by means of rendering the electrostatic latent image visible. Alternatively, the image coverage measurement means may be arranged so as to measure image coverage by utilization of an image signal to be used for writing an electrostatic latent image.
Any means may be selected as the image concentration detection means for detecting the concentration of an image, as required: for example, means for detecting the concentration of a reference image for use in detecting a concentration formed on the image carrier or means for detecting the proportion of toner contained in the developing agent stored in the developing unit, so long as the image concentration detection means can directly or indirectly detect the concentration of an image.
The toner supply control means may selectively determine when to supply the determined amount of toner or a unit in which toner is to be supplied.
The expression xe2x80x9camount of toner supplyxe2x80x9d means all the types of quantity of toner supply which enable direct or indirect ascertainment of the amount of toner supply. In view of ease of handling, the amount of toner supply is expressed by a time period during which the toner supply means is to be activated, or the number of rotations of drive elements of the toner supply means.
The toner supply history storage means enables storage of a history concerned with the amount of toner which had been supplied during a certain period of time, for the purpose of ascertaining the state of toner consumption.
Preferably, information about the amount of toner which had really been supplied over at least a certain period of time is stored as history information. From the viewpoint of more accurate ascertainment of the amount of toner to be supplied essentially, the toner supply history storage means preferably has means for storing the amount of deficient toner supply which should have been supplied in the past but has not been supplied, the amount of excessive toner supply which should have been reduced in the past but has not been reduced, or the result of addition of the amount of deficient toner supply and the amount of excessive toner supply.
The expression xe2x80x9cthe excessive consumption of tonerxe2x80x9d determined by the toner consumption state determination means, for example, a case where high-coverage images are continuously formed. In such a state, toner is excessively consumed to such an extent that ordinary toner supply cannot keep pace with consumption of toner. As a result, irregularities in the electrification characteristic of toner cause a failure in picture quality, or a deficiency of toner induces print failures.
A determination as to whether or not consumption of toner is excessive is basically made on the basis of whether or not the quantity of toner consumed during a predetermined period of time exceeds a predetermined amount. Various situations lead to excessive consumption of toner. From the viewpoint of more correct determination of excessive consumption of toner, excessive consumption of toner is preferably determined in several phases on the basis of a history concerning the amount of toner supplied during different periods of history; for example, the phase of excessive consumption of toner abruptly arising in a short period of time; the phase of excessive consumption of toner gradually arising over a certain period of time; and the phase of excessive consumption of toner arising during an intermediate period of time.
The development property stabilization means may be embodied as any means, so long as the means interrupts formation of an image when it is determined that toner is excessively consumed, thus stabilizing the development property of a developing agent. Stabilization of development property of a developing agent is typically effected by means of the toner supply means supplying toner and the developing unit agitating and blending a developing agent. Alternatively, stabilization of development property of a developing agent may be effected by means of only the developing unit agitating and blending a developing agent.
From the viewpoint of avoiding a useless operation, the development property stabilization means preferably detects the concentration of an image through use of the image concentration detection means. In a case where the concentration of the image does not reach a target concentration level, the toner supply means preferably supplies toner, and the developing unit 3 preferably agitates and blends a developing agent or temporarily agitates and blends a developing agent.
In a case where emphasis is placed on stabilization of a development property, if the toner consumption state determination means determines that toner is excessively consumed, the development property stabilization means interrupts formation of an image. Subsequently, the image concentration detection means detects the concentration of an image. In a case where a target image concentration level is achieved, the developing unit preferably temporarily agitates and blends a developing agent.
Preferably, the toner supply history storage means has means for storing the amount of deficient toner supply which should have been supplied in the past but has not been supplied.
Preferably, the toner supply history storage means has means for storing the amount of excessive toner supply which should have been reduced in the past but has not been reduced.
Preferably, the toner supply history storage means has means for storing the result of addition of the amount of deficient toner supply which should have been supplied in the past but has not been supplied and the amount of excessive toner supply which should have been reduced in the past but has not been reduced.